Lewes, Delaware (CNN) - Christine O'Donnell says "it's a shame" that a major conservative grassroots organization is deciding not to endorse her conservative bid for Delaware's Republican Senate nomination.

Monday two of the top members of FreedomWorks declined to endorse O'Donnell, a conservative commentator who is facing off against moderate Republican Rep. Mike Castle for their party's nomination in Tuesday's primary.

At a Christian Science Monitor Breakfast in Washington DC, FreedomWorks President Matt Kibbe said "we've stayed out of that race because we're not convinced that Christine O'Donnell can win."

And when asked by reporters at the breakfast whether "it's better for Republicans to lose with a Tea Party-backed candidate than to win with a mainstream Republican candidate," FreedomWorks Chairman and former House Minority Leader Dick Armey said "no."

FreedomWorks is a nonprofit conservative organization based in the nation's capitol that helps train volunteer activists and has provided much of the organizational heft behind the Tea Party movement.

O'Donnell, a conservative commentator and marketing consultant who ran unsuccessfully for the Senate in 2006 and 2008, has been backed by the Tea Party Express, one of the best known national Tea Party organizations. The group is running television and radio ads in support of O'Donnell and says they'll spend more than $250,000 to help her primary bid. The group is very critical of Castle, a former two-term governor and nine-term state wide congressman.

Former Alaska Governor Sarah Palin has also endorsed O'Donnell and has recorded a robo-call to urge Delaware Republicans to vote for the candidate.

Speaking to CNN Monday afternoon about the FreedomWorks announcement, O'Donnell said "it's a shame. No one thought I could win a primary and you know on the eve of Election Day we are ahead. I'm not taking anything for granted, there's still a lot of undecided votes. But no one even thought we could get this far. And I truly believe we've got a winning message."

O'Donnell is confident that she can pull the upset and win on Tuesday, saying that "once we get past the primary we're gonna work very hard to go in all three counties as often as possible, reach the voters and present my message to them."

And O'Donnell's confident can win support of some Democrats in the general election if she wins the primary.

"Absolutely we've got Hillary Democrats working on our team right now. We've got a broad coalition of support that are behind me. What you see in the voter who is rallying behind my campaign are the people who are frustrated with politics as usual. The people who want their voice heard again. And they want to be part of the system. Those are the folks who are behind me. And that anti-incumbent, anti-establishment frustration resonates through both political parties," said O'Donnell.